With the emergence of various services such as a video service, bandwidth pressure on a transport network becomes higher, and costs of using a WDM (Wavelength Division Multiplexing, wavelength division multiplexing) technology also becomes higher. Variable spectrum, as a network technology that can flexibly configure an optical fiber spectrum resource according to a signal rate and spectral width, may be used as an important technology to effectively alleviate the bandwidth pressure.
On a conventional WDM network, regardless of the number of spans to pass through and transmission distance, each optical path at a same rate must be transmitted according to a spectral bandwidth stipulated by the ITU-T (International Telecommunication Union-Telecommunication Standardization Sector); for the robustness and upgradability of a system, networking is designed according to the highest OSNR (Optical Signal-to-Noise Ratio, optical signal-to-noise ratio) standard, that is, a same modulation mode and spectral bandwidth are adopted for all paths; as a result, the OSNR of a short optical path with the small number of spans is far greater than the OSNR that is actually required, wasting a spectrum resource of the system. A variable spectrum technology may flexibly select an appropriate modulation mode and spectrum resource according to the length of an optical path and the number of spans to pass through, so as to achieve an objective of more effectively utilizing a spectrum resource of a line and improving the spectrum efficiency. How to implement hitless switching of a modulation mode to ensure normal sending and receiving of service data is a key point of the variable spectrum technology. Currently, when a modulation mode is switched, a manner of inserting overhead bits by a sending end is adopted to instruct a receiving end, so as to implement sending and receiving of service data. However, this manner requires protocol processing at both the receiving end and sending end, resulting in complex implementation and low reliability.